how can i fix my boiler?

hi there how are you i have a problem with my boiler and was doing a bit of research on the net, i have a valliant turbomax plus 828e which is about 6 years old. The problem im having is with the ch the boiler only heats up stairs and not downstairs also the boiler will come only for about 20 - 30 sec and go off the temperture rises on the boiler digi screen which when it drops the boiler will kick up again the only thing that is confusing me is why does the boiler switch of at about 75 degrees when all the radiators arent warm hope this is making sense.ive tried various things such as bleeding all the rads and also shutting all the rads off bar 1 downstairs, it gets warm but not hot as it used to ive repeated this process on all the rads ive open the screw on the pump there was a little bit off air in there not much which i took.i would be grateful if you can advise me on what i should do next if youve got any idea of what it could be will be appreiated.thanks for taking the time to read my problem jay p.s im not having any problems with the hot water (touch wood)